The invention relates to aircraft ground proximity warning systems and more particularly to excessive terrain closure rate warning systems.
In the prior art, as specifically represented by Astengo U.S. Pat. No. 3,715,718; the application of Bateman Ser. No. 480,727, filed on June 19, 1974, entitled, "Ground Proximity Warning Instrument"; and the application of Bateman et al. Ser. No. 556,022, filed Mar. 6, 1975, entitled, "Terrain Closure Warning System With Altitude Rate Signal Conditioning," all of which are assigned to the assignee of this application, a signal, derived from a radio altimeter, that represents the rate of change in the aircraft's altitude above ground is compared with a signal from the radio altimeter indicating the aircraft's altitude above the ground to generate a warning signal when the terrain closure rate is excessive for the aircraft's altitude. In the prior art systems, the warning signal remains on as the aircraft continues to approach the terrain and switches off as soon as either the aircraft begins altitude by climbing or when the terrain begins to slope downwardly from the aircraft's flight path.
There are, however, situations in which it would be useful from a safety standpoint to insure that the aircraft has climbed a predetermined distance before leveling off after having received a terrain closure warning signal. This most commonly occurs where the aircraft is traversing terrain having a series of peaks and valleys with slopes that are sufficiently steep to trigger short warnings. Assuming the aircraft is in a level flight over undulating terrain that is gradually increasing in altitude, the aircraft may experience one or more short warnings. However, since each upward slope is followed by a downward slope, the crew of the aircraft responding to the warning signal by observing the radio altimeter would be under the impression that his ground clearance was increasing because the aircraft at that point would be traversing a downward slope. Because such observations of the radio altimeter would appear to indicate that the aircraft is not in actual danger, the aircrew may elect to maintain level flight, thus reducing the aircraft's chances of responding quickly enough to a warning in order to prevent the aircraft's impacting a slope that rises above the aircraft's flight level.
A careful study of a number of accidents in view of the type of terrain that tends to increase in altitude, has indicated that in a large number of cases if the aircrew were to take immediate action on the first short warning and increase the aircraft's altitude by 200 feet, a significant number of potential accidents could be avoided.